1. Field of the Invention
The present invention relates to a method for producing a spark plug for an internal combustion engine and to a spark plug manufactured by the method, and more particularly, to a method for producing a spark plug having an outer electrode in which an outer electrode chip is joined to an outer electrode base material via an intermediate member and to a spark plug manufactured by the method.
2. Description of the Related Art
Conventionally known spark plugs include spark plugs having a center electrode and an outer electrode in which an outer electrode chip is joined to an outer electrode base material via an intermediate member. For example, Patent Document 1 and Patent Document 2 disclose such a spark plug.
In Patent Document 1, the outer electrode of a spark plug is produced as follows. That is, a chip-shaped electrode material (outer electrode chip) which is resistant to spark-induced ablation is joined to an end of a bar-shaped member (intermediate member) made of a base metal resistant to corrosion, by means of TIG welding (tungsten inert gas welding) or laser welding. Subsequently, the corrosion-resistant base metal member (intermediate member) is cut to an appropriate dimension. The flat surfaces of the corrosion-resistant base metal member (intermediate member) and the outer electrode (outer electrode base material) are brought into contact with each other, and welded together through resistance welding, whereby the outer electrode is formed (see claims and other sections of Patent Document 1).
In Patent Document 2, the outer electrode is produced as follows. That is, an intermediate member having first and second parallel surfaces is fabricated in advance, and a chip (outer electrode chip) is laser-welded to the first surface of the intermediate member. Subsequently, the second surface of the intermediate member and a joint surface of the electrode base material (outer electrode base material) are brought into contact with each other, and are welded together through resistance welding, whereby the outer electrode is formed (see claims and other sections of Patent Document 2). The resistance welding between the intermediate member and the electrode base material (outer electrode base material) is performed by supplying current thereto while pressing a circumferential edge portion of the intermediate member by means of an electric resistance welding machine (see FIG. 4 and its description in Patent Document 2).
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. H8-298178
[Patent Document 2] Japanese Patent Application Laid-Open (kokai) No. 2004-134209
3. Problems to be Solved by the Invention
However, when a spark plug produced in accordance with Patent Document 1 or 2 is subjected to a thermal cycle test in which a thermal cycle of heating the spark plug to a high temperature and then naturally cooling is repeated a large number of times, a large gap may be produced between the intermediate member and the outer electrode base material at a radially central portion thereof. Such a gap results in deteriorated reliability of the joint between the intermediate member and the outer electrode base material. Further, in some cases, a hollow portion is generated in a fused metal alloy portion between the outer electrode chip and the intermediate member at an outer periphery of the fused metal portion.
Conceivably, the reason why a large gap is produced between the intermediate member and the outer electrode base material at a radially central portion thereof is as follows. That is, when the intermediate member and the outer electrode base material are resistance-welded, a circumferential edge portion of the intermediate member is pressed against the outer electrode base material. Therefore, at that circumferential edge portion, the intermediate member and the outer electrode base material are mixedly fused and welded without fail. However, since no pressure is applied to a radially central portion of the intermediate member, at that central portion, the intermediate member and the outer electrode base material are not reliably welded in some instances. Therefore, when a thermal cycle test is performed, due to thermal stress produced at that time, a large gap tends to form at a radially central portion at which the intermediate member and the outer electrode base material are not reliably welded.
Further, the reason why a hollow portion is formed in the fused metal portion between the outer electrode chip and the intermediate member is as follows. That is, when a large gap is present between the intermediate member and the outer electrode base material at a radially central portion thereof as described above, transfer of heat from the outer electrode chip to the outer electrode base material deteriorates. Therefore, during a thermal cycle test, the fused metal portion between the outer electrode chip and the intermediate member is exposed to high temperature. As a result, the fused metal portion is subject to high-temperature oxidation, so that an alloy which constitutes the fused metal portion is gradually consumed, and a hollow portion is formed. Further, in the spark plug of Patent Document 1, in order to secure a sufficient joining area between the intermediate member and the outer electrode base material, the entire intermediate member must be increased in size. In contrast, in the spark plug of Patent Document 2, the intermediate member has a stepped structure; i.e., has a cylindrical columnar portion having a smaller diameter and a flange portion having a larger diameter. Thus, it is possible to adjust the diameter of the flange so as to secure a sufficient joint area, while matching the diameter of the cylindrical columnar portion with the diameter of the outer electrode chip. However, since the intermediate member is configured such that the cylindrical columnar portion disappears during laser welding, and only the flange portion is left after the laser welding, the incident angle of a laser beam during the laser welding is likely to be restricted by the flange portion.